Planar ohmic contacts to Al<inf>0.45</inf>Ga<inf>0.55</inf>N/Al<inf>0.3</inf>Ga<inf>0.7</inf>N high electron mobility transistors

We present a low resistance, straightforward planar ohmic contact for Al0.45Ga0.55N/Al0.3Ga0.7N high electron mobility transistors. Five metal stacks (a/Al/b/Au; a = Ti, Zr, V, Nb/Ti; b = Ni, Mo, V) were evaluated at three individual annealing temperatures (850, 900, and 950◦C). The Ti/Al/Ni/Au achieved the lowest specific contact resistance at a 900◦C anneal temperature. Transmission electron microscopy analysis revealed a metal-semiconductor interface of Ti-Al-Au for an ohmic (900◦C anneal) and a Schottky (850◦C anneal) Ti/Al/Ni/Au stack. HEMTs were fabricated using the optimized recipe with resulting contacts that had room-temperature specific contact resistances of ρc = 2.5 × 10−5 cm2, sheet resistances of RSH = 3.9 k/, and maximum current densities of 75 mA/mm (at VGATE of 2 V). Electrical measurements from −50 to 200◦C had decreasing specific contact resistance and increasing sheet resistance, with increasing temperature. These contacts enabled state-of-the-art performance of Al0.45Ga0.55N/Al0.3Ga0.7N HEMTs.